Diffusion coating for metals



United States Patent Office 3,257,230 Patented June 21, 1966 3,257,230DIFFUSION COATING FOR METALS Richard L. Wachtell, Scarsdale, and RichardP. Seehg, Hartsdale, N.Y., assignors to Chromalloy American Corporation,a corporation of New York No Drawing. Filed Mar. 24, 1964, Ser. No.354,440 14 Claims. (Cl. 117107.2)

This application is a continuation-in-part of co-pending applicationSerial No. 807,025, filed April 17, 1959, and now abandoned.

- This invention relates to the diffusion coating of metal articles forthe production thereon of an outer coating or layer of enhancedoxidation and corrosion or erosion and thermal shock resistance at hightemperatures, as well as other enhanced surface characteristics, inwhich the article to be coated is heated as embedded in or otherwise insurface contact with a powdered pack or mixture including a metalliccoating material, and, more particularly, to the production of suchdiffusion coatings where the coating pack or mixture includes at leastone ingredient other than the primary coating material for controllingthe rate at which the coating material is presented to the surface ofthe article to be coated for diffusion thereinto.

As will be understood, there is now known a wide variety of differentprocesses and techniques for producing a diffusion coating or layer intoor on the surface of metal articles. Some of these well-known techniques(and those generally of the character to which this inventionparticularly relates) involve embedding the article to be coated (orotherwise covering the surface thereof)in powdered coating packincluding a powdered source of the coating material (with or withoutadmixture with powdered inert filler) and .a vaporizable carrieringredient '(such as a heat-volatile halide),'and heating the thusembedded article and pack in a sealed retort (or other controlled andgenerally non-oxidizing atmosphereyto an elevated temperature at whichthe carrier material will vaporize and/or otherwise react with orfunction as a carrier for transporting the coating material from orthrough the powdered pack to the surface of the article to be coated fordiffusion or other reaction thereat. I Generally speaking, the variouschemical reactions involved (e.g.', between the carrier and the coatingmaterial, between the coating material -and the metal or othercomponents of the article to be coated, among whatever ingredients arein the pack, between coating material and whatever inter-metallics oralloys may have already been formed at or in the surface of the articlebeing coated, etc.) occur more or less simultaneously during the heatingtreatment and are mostly of a reversible nature, so that the net resultof the coating step and the chemical reactions therein involved maydepend predominantly upon the various equilibria achieved. That is, aswill be understood, under certain temperature conditions and withcertain reactive carrier materials, ingredients in the coating pack maybe inclined to combine with each other .at the same time (and, perhaps,even at the same rate) as one or another thereof may diffuse into thesurface of the metal article; while (if the equilibrium conditions areappropriate) some portion of the metal from the article itself, or oneor another component thereof, may also diffuse out of the article andinto the pack ingredients.

Although it has been possible in the past to select certain specificingredients or operating conditions, on a more or less empiric-a1 basis,for obtaining some commercial effectiveness of such pack coatingtechniques, it

is to be understood that a variety of limiting factors may be present incommercial operations with different coating materials and differentmetal articles to be coated which are actually inconsistent orincompatible with the factors or operating conditions which may maximizeor control the equilibria or reaction rates most favorable to achievingthe desired efficient coating or recovery of a particular coatingmaterial into a particular metal article.

For example, as will be understood, the chemistry within the coatingpack may determine, for a specific temperature, the rate at whichcoating material is delivered to the surface of a coated article fordiffusion thereinto, yet this delivery rate may not necessarily bedirectly related to the rate of diffusion of the same coating materialinto the article after having penetrated the surface thereof. Under suchcircumstances, the situation may arise where a particular hightemperature is adequate for forcing a particular coating material tobegin to diffuse into the surface of the article, but yet so high, withrespect to the'diffusion rate of the material inside the article, thatthe coating material will be driven on into the center of the articleand away from the surface thereof under prolonged exposure to the samehigh temperature. Such a result, of course, may not produce the desiredsurface coating, and may even result in a situation where too littleconcentration of the coating at the surface is obtained becausetemperatures high enough to decompose the pack ingredients for diffusionin the first place may be sufficiently high to drive the diffusedcoating material too far into the interior of the article being coated.

Conversely, as will be understood, preliminary diffusion of some coatingmaterial into the article surface may so inhibit the penetration ofadditional coating material sufiicient to form thedesired thickness ofcoating as to be incompatible with a treating temperature consistentwith forcing the reversible reactions in the coating pack toward thedesired equilibrium result. As another possibility, the composition ofvthe article being coated may be subject to crystallographic ormetallog-raphi-c changes at different-temperatures under circumstanceswhich are completely unrelated to the reaction rates of the variousreactions in the coating pack itself so that treating temperaturessufficient to induce diffusion of the coating material toward or intothe surface of the article being coated are too low to achieve thedesired temperature or metallographic thermal condition of the articlebeing coated to receive the coating material in the desired manner.Indeed, with some low melting articles, and utilizing treatingtechniques in which the retort itself is sealed by preliminary meltingof the fusible seal and/or where a certain threshold temperature isnecessary for breakdown of the coating pack, the article to be coatedmay appr-oximate-a molten condition before thermal conditions can beestablished promoting the desired diffusion from the coating materialsin the pack.

As merely an illustrative example of such difficulties as may beencountered under commercial operations, one may note such situations aswhere it is desired to diffusion coat aluminum into the surface of ametal article including a substantial proportion of, for example, copperto form only a protective surface coating or layer, which layer, forultimate protection, it is desired to have formed as a particular one ofa variety of possible copper aluminides. It has been found that aluminumis'readily and rapidly diffused into such copper-containing articles ata relatively moderate treating temperature, with the diffusion rateincreasing rapidly as the temperature is increased, while the particularcopper aluminide formed in the metal article also depends upon thetreating temperature. Under these circumstances, if the treatingtemperature is controlled to no more than will force diffusion ofaluminum at a fairly moderate rate (so as to avoid deep diffusion of thealuminum and retain a surface coating or layer), it may be found thatthe particularcopper aluminide formed in the coating is not the onedesired; whereas, on the other hand, if the treating temperature israised to the point Where the desired particular copper aluminide isformed in the surface of the article, such higher temperature mayproduce an aluminum diffusion rate greatly in excess of that desiredand/ or so as to produce too deep penetration of the aluminum into thearticle being coated.

Whereas the foregoing example is advanced here merelyas illustrating atype of problem encountered in the commercial utilization of thediffusion coating techniques, it is one frequently encountered in thefield of diffusion coating of so-called superalloy materials (i.e., hightemperature resistant alloys having substantialproportions of nickel,cobalt, chromium, etc), in which field diffusion coatings becomeincreasingly important for the oxidation protection of such superalloysas they become increasingly necessary for use in jet engine, rocketmechanisms, and similar high temperature applications.

Generally, such problems of too rapid transfer rate of the coatingmaterial from the pack ingredients to the surface of the article to becoated may be frequently encountered in treating high temperature orcomplex alloys, on the one hand, or articles with which the coatingmaterial and article may-form low-melting complexes or systems, on theother hand, and particularly in situations Where it is desired toutilize a more or less standardized-commercial operating technique forthe application of various different coating materials to a wide varietyof metal articles of greatly different chemical and metallurgicalcharacteristics. In such situations, for example, the particulartemperature levels which must be obtained to instigate and maintain theinitial reactions between the particular carrier material and the sourceof coating material in the pack and/or the thermal conditions andtemperature levels which must be maintained in order to drive suchreactions to completion within any commercially tolerable length of timemay have no direct relation at all (indeed, they usually do not have)with the chemical characteristics and/ or diffusion rates of the coatingmaterial at or into the surface of different articles being coated.

Similarly, as will be understood, the complexity of the alloy orinter-metallic surface of the article being coated (either originally orafter a portion of the coating material has been diffused thereinto)adds a further complication or additional factor, at least from thestandpoint of predicting a set of operating conditions which will in allcases produce a satisfactory diffusion coating and/or permittingconditions which will drive one of the various possible reactionsdesirably toward the particular result desired. Especially is this truewhen (as in most cases) there is a variety of different intermetalliccompounds which may result from the combination of a particular coatingmaterial with a par-' ticular ingredient in the article and undercircumstances where a variety of such different compounds may beproduced in the same article at different temperature levels and/ordepending upon other operating conditions which may rarely be socompatible as to permit maximizing the operating conditions for one setof reactions without upsetting diffusion rate and temperature conditionswithin the article which might be desired to produce a particularintermetallic resultant.

According to this invention, however, a variety of techniques andcompositions is provided for ready application to or inclusion in thediffusion coating pack itself for controlling or inhibiting the rate oftransfer of the coating material (as accomplished by a volatile carriermaterial) from the pack ingredients to the surface of the article beingcoated for correlating such rate of transfer, even independently of theinherent characteristics of the coating material, so that suchtransferrate or presentation rate is predictably correlated with or adjusted tothe article being coated and the particular compositions and ingredientsthereof so as to assure that,

at commercially effective temperature levels, the particularintermetallic resultant desired Will be formed in the article surface,to the exclusion of other possible resultants, and/or that presentationof the coating material to the article surface in view of the diffusionrate thereof into the article neither inhibits nor excessivelyaccelerates diffusion or reactions within the article inimical to orinconsistent with the formation at the surface thereof of a diffusioncoating layer of the desired thickness, composition, and protectivecharacteristics.

To accomplish such ends in accordance herewith, one or more extraingredients are added to or included in the coating pack for combinationor interaction with the material to be coated and/ or the carriercomponent of the pack for forming a rate-controlling intermetallic orother compound with the coating material or otherwise altering thenormal transfer rate of such material through the pack at the particulartemperature being used, with such extra ingredients usually beingmetallic (by which term is meant to include elements such as silicon orboron as well as elements usually considered as metals) and even mayitself not be diffused into'the metal article to form a significant orcontributing part of the ultimately desired coating thereon.Furthermore, the selection of the particular extra ingredient hereof,with due regard to the particular car- 'er component being utilized, isspecifically correlated to the metallic or chemical characteristics ofthe article being coated and the diffusion or reaction rates of thecoating material therein and the variety of possible intermetalliccompounds which may be formed between the coating material andcomponents of the coated article of to the end of regulating orcontrolling the transfer rate of the coating material from the-pack tothe surface of the article being coated to conform to predeterminedtemperature and other operating conditions so as to provide in thefinished coating the particular intermetallic or alloy or othercomposition desired notwithstanding the fact that the normal or inherentforming of such desired composition may be quite inimical to orincompatible with the predetermined temperature or other operatingconditions desired and/ or, indeed, impossible of formation under anyoperating conditions without the provision of the added rate-controllingingredients.

With the foregoing and additional objects in view, this invention willnow be described in more detail, and other objects and advantagesthereof will be apparent from the following description and the appendedclaims.

Although, as noted above, this invention and the teachings thereof areapplicable to a wide variety of different coating situations utilizingdifferent coating materials and different compositions of articles to becoated, it may generally be convenient initially to describe techniquesand compositions embodying and for practicing this invention asparticularly, although merely illustratively, applied to forming anessentially aluminide coating on high-nickel and high-cobalt superalloysand utilizing a coating pack containing, in addition to the aluminum tobe coated, an inert filler, a volatilizable halide material such asammonium fluoride as the carrier component, and metallic chromium as theextra rate-controlling ingredient.

As will be understood, although such superalloys inherently exhibitphysical properties within a desired range, particularly when subjectedin use to extremely high temperatures, the oxidation resistance and/orcorrosion resistance of the surface of such alloys, particularly whensubjected for prolonged times to such high temperatures, maybe less thandesired for prolonged or severe use. If it is attempted to increase theoxidation and corrosion resistance of the surface of such high-nickel orhigh-cobalt alloys by diffusion coating thereinto of a metal such aschromium according to contions to which the article may be subjected inuse.

,ventional diffusion coating techniques, an enhancing of the oxidationand corrosion resistance may be experienced, but, less than the optimumwhich may be desired. Alternatively, attempting to enhance the corrosionresistance of such alloys by applying thereto a surface coating ofaluminum as by an aluminum dip or similar conventional technique, theoxidation-resistant aluminum coating may be found to have less thanoptimum per forming qualities, particularly under severe thermal shockand thermal deformation to which such articles may be subjected in use.

Nevertheless, a diffusion coating of aluminum produces, at the surfaceof ,such alloy articles, a layer of substantially enhanced resistance tooxidation and erosion, as well as a casing of good and uniform adherenceto the article, even during thermal shocks and deforma- This lattercharacteristic is ofparticular importance in the production ofoxidation-resistant coatings on high temperatu're alloys subjected tosevere thermal conditions because, if the supposedly oxidation-resistantcoating or outer casing is not maintained uniformly continuous andfirmly adhered to the article during thermal deformations thereof,fissures or other discontinuities may occur in the oxidation-resistantcoating as a result of thermal shock, which fissures, once havingoccurred, readily present easy access to the base metalof the articlefor oxidation corrosion or erosion thereof.

Considering the aluminum coatings of this particular illustrativeexample, especially satisfactory results are achieved with base metalalloys containing a substantial or preponderant proportion of nickel orcobalt and chromiu'me.g., such alloys as are particularly formulated forhigh temperature use and having physical properties and a useful life asdesired when subjected for prolonged duration to both very hightemperatures 7 and to severe thermal shocks and rapid changes oftemperature over wide ranges. Whereas such superalloys, as wellunderstood, may have small amounts (e.-g., usually less than 10%) ofiron, they are composed primarily of a substantial proportion ofchromium (e.g., about 10%- 20%), with at least about 50% or more of thecomposition being made up of nickel or cobalt and/or mixtures thereof.

Merely as illustrative of the types of high temperature alloys for whichthis invention is particularly adapted, one may note a commercialnickel-base alloy. sold by Utica Metals Division of Kelsey-HayesCorporation under the designation Udimet 500 and a highcobalt-containing alloy commercially manufactured and sold by theHaynes-Stellite Division of Union Carbide under the designation X-40.Such alloys have approximately the following compositions (according tothe respective manufacturers specifications) in weight percent:

High nickel alloy: Percent Carbon 0.12 Chromium 19 Cobalt 19 Iron 1Molybdenum 4 Aluminum 3 Titanium 3 Nickel balance High cobalt alloy:Percent Carbon 0.50 Chromium 24.5 Nickel 10.5 .Tungsten 7.4

Iron 1 Cobalt balance Also illustrative of the type of superalloymaterials with which satisfactory results are achieved with thisinvention are the alloy high temperature steels and alloys having aboutequal proportions of nickel, cobalt and iron.

Also as further illustrative of the procedures and coatbe noted thatarticles of such high temperature alloys for which this invention isparticularly adapted are satisfactorily coated by procedures includingembedding the article to be coated in a dry powder pack including aninert mineral material, a source of the metallic elements to bediffusion coated, and a source of a vaporizable or diffusible halogen.As embedded in such a pack (preferably contained within a metalcontainer the seams of which are sealed by a fusible material such as alow-melting silicate to prevent excessive escape of the diffusingmaterials during heating and also excessive introduction of air into thepack during cooling), the articles are heated to a substantialtemperature for a number of hours to cause diffusion coating of thedesired metals, in conjunction with the elemental halogen, into thesurface of the articles being treated.

Satisfactory results have been achieved according to this invention inso coating articles formed from any of the illustrative alloys mentionedabove with the use of a coating pack comprising, as illustrative of thisinvention,

.approximately 69% alumina as the inert mineral, 22%

chromium metal and 8% aluminum metal as the metals to be diffusioncoated, and 1% elemental iodine as the vaporizable halogen, theforegoing percentages being by weight. With such a pack, enclosed in acontainer in known manner for producing diffusion coating of variousmaterials on various metallic alloys, satisfactory results have beenachieved according to this invention by heating the articles ofhigh-nickel or high-cobalt content in the pack for from 4 to 20 hours attemperatures of from about 1800 F. to 2100" F.

In connection with the foregoing ranges, it should be noted that, ifthinner coating layers or cases are desired (e.g., where severe thermalshock and deformation of the articles are anticipated), the diffusioncoating is carried out, preferably, at the lower temperatures and/or forshorter times within the foregoing ranges, and, where thicker cases maybe desired (e.g., where oxidation and erosion resistance is of moreimportance than resistance to the possible disruption of the coatinglayer or casing by thermal shock), the diffusion coating step isconducted at higher temperatures and/or for longer times, therebyappropriately controlling the thickness of the diffused coating layer orcasing produced in accordance with this invention.

As will be understood by men skilled in the art of the diffusion coatingof metals, the proportions of materials, and the materials themselves,suggested in the above mentioned pack may be varied over fairly wideranges. Thus, the proportion of inert filler is not critical and,although alumina is a preferred filler material for such a pack,

other inert fillers are satisfactory. Similarly, other halo gen sourcesthan the elemental iodine mentioned above give satisfactory resultsprovided they are capable of producing a vaporized halogen at thetemperature and under the conditions of operation. In addition toelemental iodine, ammonium iodide and ammonium fluoride are'particularly satisfactory. Also as noted, the time and temperatureconditions of the diffusion coating step may, to some extent, be varieddepending upon the thickness of the coating desired. Satisfactoryresults have been achieved with coatings of the order of .001 to .002inch,

although even thinner coatings give substantially enhanced oxidation andother protection to the base alloy,

and, for some applications, thicker coatings may be} sisting thermalshock and thermally induced dimensional variations to which the finishedarticle will be subjected in use. Also the aluminum coatings, accordingto this invention, in addition to being oxidation-resistant, alsoexhibit good resistance to chemical corrosion attack, other thanoxidation, at high temperature, as well as resistance to thermallyinduced metallurgical changes even after prolonged exposure to very hightemperatures. Such coatings also exhibit good continued adherence to thecoated article despite rapid and severe temperature changes to which thearticle may be subjected in use.

Purely as illustrative of some of the foregoing advantages, certaincomparative test data may be noted. For example, an article composed ofone of the highcobalt alloys to which this invention relates exhibits asatisfactory life of no more than about hours when continuously exposedto a temperature of about 2000 F., and the same alloy with a protectivecoating of aluminum alone exhibits a satisfactory life of only about 20to 30 hours under exposure to the same high temperature, with a coatingof chromium alone affording less protection than the aluminum coating.By contrast, the same highcobalt alloy having a diffusion coating ofaluminum according to this invention exhibits a satisfactory life of 150hours or more under continuous exposure to a temperature of 2000 F. Oneof the high-nickel alloys, without a protective coating of eitherchromium or aluminum, may exhibit a satisfactory life of up to 100 hoursat 2000 F., with little or no extension of such life when the alloy isprovided with a coating of aluminum alone or chromium alone. Bycontrast, however, satisfactory life of 150 hours or more at 2000 F. isachieved by such a high-nickel alloy when provided with an aluminumdiffusion coating according to this invention.

Similar surprisingly enhanced results may also be noted with regard tothermal shock tests, and this characteristic of coatings embodying thisinvention may, for many purposes, be at least as significant asresistance to continued high temperature exposure. That is, considering,for example, parts of gas turbine airplane engines which operate atextremely high temperatures, but not necessarily for long periods ofcontinuous operation, the internal parts of such gas turbine enginessubjected to high operating temperatures must also be able to withstandthe thermal shock incident to a sudden increase in temperature in but afew minutes all the way from the ambient temperature outdoors in thewinter to the extremely high operating temperature of the engine uponstarting of the engine, and, shortly thereafter, upon stopping, a severefast cooling from the operating temperature back down to the ambienttemperature. One of the high-nickel alloys was observed to Withstandabout 130 rapidly repeated cycles of heating from room temperature up to1850 F. to 2000 F. and cooling back down to room' temperature withoutfailure, while a similar article composed of a high-cobalt alloywithstood about 260 cycles of the same testing.- Neither of thesearticles was notably improved in this respect by a coating of aluminumalone, and only slightly improved by a coating of chromium alone. Bycontrast, however, when coated according to this invention with achromiumaluminum coating, both the high-nickel and high-cobalt partswithstood at least 1000 of such heating and cooling cyclessatisfactorily and without failure.

Satisfactory results are achieved in accordance herewith utilizingwidely varying proportions of the materials in the coating pack. Forexample, it is to be recognized that the rate controlling or inhibitingmechanism of a material such as chromium in an aluminizing pack may berelated to the preliminary formation within the pack of a chromiumaluminide at temperatures perhaps below the ultimate treatingtemperature. Although both chromium and aluminum may be initiallypresent in the pack in elemental form, chromium aluminides may be formedduring heating the pack and preferentially or prior to transfer ordiffusion of the aluminum component into the surface of the articlebeing coated. Thus, if a relatively stablechromium aluminide be formedin the pack prior to transfer of aluminum to the surfaceof the articlebeing coated, the medium from which aluminum is to be transferred to-thearticle being coated is not metallic or elemental aluminum, but thechromium aluminide, which may require decomposition in order to diffusealuminum into the surface of the article, especially if the diffusionrates (or even the possibility of diffusion) of the chromium aluminideis not thermodynamically effective or achievable at the particulartreating temperature.

It has been found, as noted above, that aluminum diffusion into thesurface of a predominantly nickel-containing article may be productiveof a variety of different nickel aluminides, with the particular oneformed being perhaps a function of the proportion of aluminum carried tothe article surface or diffused therein at the particular operatingtemperature. If the particular aluminide desired is one containing lessthan the maximum amount of aluminum, formation thereof may not occur ifaluminum from the pack be too rapidly presented to or available in thearticle surface. If the treatment is maintained at a sufficiently hightemperature and prolonged to achieve a desired thickness of coatinglayer, too rapid transfer of aluminum from the pack to the surface ofthe article (or diffusion from the surface on inwardly of the article)may occur to form a lower melting high-aluminum aluminide rather thanthe one desired. By including a portion of a material, such as chromium,in the pack to form with the aluminum therein a preliminaryintermetallic the diffusion of which into the surface of the article canonly occur at a diminished rate (or, perhaps, cannot occur at all) atthe desired treating temperatures, the availability or transfer of thealuminum component in the pack for diffusion into the surface of thearticle is readily inhibited or controlled so that the temperaturelevels or other thermodynamic conditions necessary to break down thepreliminary chromium aluminide sufficiently for aluminum to be diffusedwill produce the desired conditions for the formation of the particularnickel aluminide desired in the surface of the article.

Utilizing an inhibiting or rate controlling component such as chromiumin an aluminum pack-for preliminary formation therein of a chromiumaluminide has been found to produce satisfactory results in thattemperatures high enough to break down the chromium aluminide fordiffusion coating of aluminum are at levels where the desired particularnickel aluminide will be formed in the surface of a nickel-containingalloy (and substantially the same considerations have been found toapply to highcobalt alloys). In fact, once a chromium aluminide isformed in the pack ingredients, it may be necessary, in addition totemperature control, to utilize an extremely aggressive carrier material(e.g., a fluoride instead of an elemental halogen or a chloride oriodide) for sufficiently aggressive attack to break up a preliminarilyformed intermetallic (somewhat along the lines as disclosed in PatentNo. 3,096,205). As will be understood, of course, the inhibitingchromium aluminide may be formed preliminarily to the actual coatingoperation and the pack composed of such intermetallic, or elementalchromium and aluminum may be added to the pack originally with thearticle therein, since the preliminary formation of the inhibitingaluminide occurs during heating of the pack and, generally, prior to anysignificant diffusion of aluminum (or simultaneously with preliminarydiffusion) in accordance with the various inherent equilibria andthermodynamic conditions of the pack ingredients.

Generally, in these particular illustrative examples in accordanceherewith, satisfactory results are achieved in the aluminizing of suchhigh-nickel and high-cobalt superalloys utilizing a variety of differentchromium-aluminum ratios or proportionings in the pack ingredients andwith or without an inert powdered filler material (such as alumina,kaolin, and similar refractory inerts) within the broad outlines of theforegoing disclosure and teachings. As Will be understood, however, toolarge a proportion of ranges.

inert filler may interfere with any diffusion coating mere-- hereof, itwill be understood that mere dilution of the diffusible ingredients ofthe pack, without other controls, may produce unsatisfactory resultsregardless of how effective it is for inhibiting the transfer orpresentation of the diffusible metallic component to the surface of thearticle being coated.

Although satisfactory results in accordance herewith are achieved in theabsence of any inert filler at all, some such filler component isusually to be preferred, and commercial results may be generallyenhanced if the active (i.e., diffusibl-e metallic) components of thepack are kept above about by weight of the pack. Conversely, andrecalling that it is the aluminum component which is to be diffused (andwhich, inherently, will both melt and diffuse at lower temperatures thanthe chromium component), some proportion of inert filler or diluent(such as alumina) is generally to be preferred in cases Where thealuminum proportion of the pack approaches or exceeds the chromiumproportions, at least in certain temperature At a given temperature, thecoating thickness achieved may generally increase as the metalliccomponents of the pack increase, yet purely economic considerations mayindicate the desirability for a substantial proportion of filler in anycase.

At treating temperatures susceptible to aluminum diffusion, many, if notmost, proportions of chromium (as the inhibiting ingredient) do notrequire an inert filler for operability, satisfactory results beingachieved without any inert filler in situations where the chromiumproportion predominates (e.g., in the range of a pack composed of 80%metallic chromium and 20% metallic aluminum). From the standpointprimarily of ready commercial applicability to high-nickel andhigh-cobalt alloys, however, chromium-aluminum proportionings in thepack ingredients above 90% chromium are not preferred (although they maybe operative) in accordance herewith primarily because of thethermodynamically engendered .difficulty of driving the aluminumcomponent of such compositions adequately or controllably into diffusionrelation with the article being coated, although such commercialdifficulties may primarily relate to sintering of the pack ingredientsaround or to the surface being coated or similar purely mechanical sideeffects which can, if desired, be controlled by other techniques. Aswill also be understood in accordance with the foregoing, excessivelyhigh aluminum proportions (predominating over chromium) in the packingredients may defeat the ratecontrolling effect of the chromium, whileexcessively high or preponderantly chromium proportions over thealuminum may defeat or inhibit the desired aluminum diffusion.

Thus, generally speaking, chromium-to-aluminum ratios (by weight) in thepack ingredients substantially less than 0.5 may include so littlechromiurn that the ratecontrolling effect thereof is either negligibleor less than to be desired as compared with a straight aluminum pack;whereas, chromium-to-aluminum ratios substantially above 4.6 may producean ultimate product having less than optimum oxidation resistance,perhaps because of too great an inhibition of aluminum available fordiffusion coating or because of some actual diffusion coating ofchromium instead of aluminum. Various determinations throughout suchproportion ranges indicate that, in accordance herewith,chromium-to-aluminum ratios Within the broad ranges of from about 0.1 to8 may be considered operative, although such chromium-to-aluminum ratiosin the pack ingredients initially beyond the range of about from 0.5 to4.6 are generally not preferred for standard commercial operations withmost types of superalloy and other articles to be aluminized inaccordance herewith. For certain applications, of course,

and especially to protect the article being coated fromother thanoxidation (as, for example, diffusion coatings to protect nickelarticles from sulphur attack, etc), satisfactory results may be achievedoutside these ranges, While yet utilizing the rate-controllingadvantages hereof and obtaining the enhanced results of this inventionin the diffusion coating of articles with one or another of theingredients of the coating pack While benefitting from the rate-controlor diffusion-inhibiting advantages obtainable in accordance herewith.

Actually, in most instances, the character of the particular coatingactually diffused into the surface of the article changes substantiallylittle throughout a very wide range of chromium-to-aluminum ratios inthe pack, although the depth of coating and other operatingcharacteristics may be predictably and controllably varied forcommercial production reasons Within suchranges. For example, even whenthe chromium component of the coating pack is as high as 92% by weight,it is primarily aluminum which is diffused intothe surface of thearticle (although such aluminum diffusion may be excessively inhibitedby reaction or merely dilution at such high chromiurn levels).Furthermore, extensive and careful analyses (by micro-probe techniques)on coated superalloys (as well as on coated pure nickel and pure cobaltarticles) indicate primarily aluminum diffusions in accordance herewith.Although some chromium aluminides (and/ or other chromium components) inthe coated surface of the article can be analytically detected, they areapparently due to the presence of substantial portions of chromium inthe base alloy itself (since they do not occur to the same extent inarticles having no chromium in the base alloy), and rarely exceedfractions of one percent even when the invention was applied to purenickel or pure cobalt articles and notwithstanding a preponderantproportion of chromium in the coating pack.

This is believed to be a further indication that the presence ofmaterials such as chromium (and capable of combining with the aluminumin the pack preliminarily) are effective in accordance herewithprimarily as a transfer-inhibiting or rate-controlling ingredient,rather than a diffusion component of the pack, at least at operatingtemperatures where the desired aluminum diffusion coating is to beobtained with such metal articles and within the proportion ranges notedabove. With excessively high chromium components in the pack, however,some little chromium diffusion may occur (and is to be noted by theappearance of minor portions of green chromium oxide after severeoxidation testing of the coated article) although such effects are bothminor and to be understood as completely ancillary to the principalrate-controllingfunction of an extra metallic ingredient (such aschromium) in the coating pack itself for the purposes of rate control oftransfer of the coating metal from the pack to the article being coatedin accordance herewith.

As will be understood, although the foregoing illustrative explanationis primarily stated in terms of a highnickel superalloy material as thearticle to be coated, substantially the same disclosure obtains withregard to high-cobalt superalloys (as mentioned above) and the formationin the diffusion coated surface layer thereof of an appropriate cobaltaluminide. For example, in addition to the Udimet 500 nickel alloy andthe X40 cobalt alloy noted above, satisfactory results have also beenachieved in accordance herewith in the provision of aluminum diffusioncoatings (and utilizing chromium as the rate-controlling ingredient ofthe pack) with a variety of other base metal alloys and articles. Forexample, among these may be noted high-cobalt alloys such as thatdesignated as WI-52 and containing approximately 63% cobalt, 20%chromium, 11% tungsten, 2% nickel, 1.5% colombium, 0.4% carbon, and thebalance iron. Similarly, satisfactory results have also been achieved inaccordance herewith on such base metal mai i terials as standardspecification SAE 1045 and SAE 52100 steels, etc., as Well as on suchmaterials as relatively pure iron (Armco iron) and commercially purenickel (A nickel).

In such coating operations, the coating ack initially contained metallicchromium metal and metallic aluminum metal approximately in the ratiosby Weight of chromium in the amount of 24 times the amount of aluminumand with the aluminum comprising about 3% to 20% by weight of thecoating pack, with a substantial amount of the pack (e.g., 60%70%) beingmade up of powdered alumina and with about 1% of ammonium fluoride asthe carrier component. One satisfactory composition is that noted abovefor all of these commercial treatments of the various metals noted-Le,69% alumina, 22% metallic chromium, 8% aluminum metal, and 1% ammoniumfluoride. In all instances, when the treating retort was opened andunpacked, the powdered pack materials readily fell away from thearticles being coated, which had a desired smooth surface and distinctcolor with coating layer or casing depths of the order of 0019-0022 onthe nickel and cobalt articles and higher on the ferrous articles.Severe oxidation testing at 2000 F. in an oxidizing atmosphere providedin a standard testing furnace indicated no failure of any of thesecoated-parts in over 85 hours of testing treatment.

As will also be understood in accordance with the foregoing, a varietyof other materials or ingredients may be included in the coating packcomposition to achieve the desired rate-controlling or inhibiting actiontherein for systems providing an aluminized diffusioncoating, as

well as for other coating materials. For example, satisfactory resultshave also been achieved in accordance herewith utilizing nickel as therate-controlling ingredient in an aluminizing pack for the coating of,not only highnickel or high-cobalt superalloys, but also such refractorymetals as molybdenum and with illustrative pack compositions such as 40%metallic nickel, 20% metallic aluminum, 40% inert filler (alumina) and avaporizable halogen carrier component such as ammonium fluoride.Similarly, such materials as cobalt, vanadium, carbon, silicon, andeven, in some cases, iron, produce satisfactory results in accordanceherewith, as rate-controlling ingredients for diffusion coating packswhere aluminum and/or various other metals are being diffused into thesurface of a variety of materials.

Both nickel and cobalt, as well as silicon and silicon carbide, producerate-controlling results in the diffusion coating of, for example,chromium as the coating material and on a variety of the superalloys andother materials noted, but especially on high-nickel alloys such asIncaloy articles containing about 13% chromium and 44% nickel with theremainder being smaller proportions of molybdenum, titanium, aluminum,iron, boron, and carbon. Also, the metals such as nickel and chromium,among other materials, are also satisfactory for inhibiting the transferrate of lower melting coating materials (e.g., copper or iron) in thetransfer and diffusion thereof from a coating pack into the surface ofan article being coated and especially where the composition of thearticle and/or the thermal characteristics of the coating material aresuch as to promote too rapid transfer or diffusion at treatingtemperatures as high as may be desired in standard commercialoperations. Similarly, in such applications as the siliconizing of steelor other ferrous alloys at temperatures high enough to produce internaldiffusion rates of silicon higher than desired, control of transfer ofsilicon from the packto the metal article is readily achieved inaccordance herewith by causing combination of the silicon with an addedingredient in the pack, and so that a treating temperature may beselected to produce the formation of a particular si-licide'in thearticle notwithstanding a normally excessive diffusion rate at suchtemperaturej Thus, generally and merely as illustrative, metals such aschromium, nickel, iron, or

12 cobalt may be considered as satisfactory rate-controlling ingredientsfor diffusion coatings of aluminum or silicon, while such materials assilicon or iron or carbon satisfactorily control the transfer ofchromium, while silicon also is useful with aluminum coatings and ironsimilarly for silicon or aluminum, etc.

It is also to be understood that the utilization of the foregoingmaterials as rate-controlling or inhibiting ingredients in the variouscoating packs mentioned or included in accordance herewith does notnecessarily exclude some diffusion of the rate-controlling ingredientitself, although usually of a minor and/ or ancillary nature, and suchoperations are generally to be distinguished from situations where anadditional ingredient is actually added to the pack purposefully to bediffused along with the primary coating metal as, for example, theaddition of carbon to the pack for carburizing or controlling thedecarburizing of a ferrous alloy (and within the article, not byreaction in the pack) during the heating treatment for diffusing someother primary coating metal into the article. Also, to be distinguishedare such situations where one or another of the materials noted may beused primarily as a getter for one or another of the possible resultantsof one or another of the concurrent reversible reactions (e.g., togetter iron halide in the diffusion coating if iron materials using ahalide carrier component) for accelerating or otherwise controllingwhich one of the several possible reactions goes farthest to completion.

Similarly, the foregoing disclosure is to be understood as relatingprimarily to rate-controlling techniques involving inhibiting thetransfer of the coating material from the pack to the surface of thearticle by chemical reaction thereof with other ingredients in the pack(and/ or, as noted, by controlling the decomposition of any resultantsof such chemical reaction by a selected degree of aggressiveness of thecarrier component), rather than to such mechanical expedients of ratecontrol as simple dilution of the pack with inert ingredients and/ orsuch techniques as may involve actual preliminary treatment of thesurface of the article being coated for inhibiting penetration thereto(or thereinto) of pack ingredients (such as the techniques disclosed inthe co-pending application of Martin Epner relating to preliminarycoating or masking of the article surface for mechanical or chemicalinterference with transfer or diffusion of coating'materials from thepack, or that of Walter Butler relating to preliminary or simultaneouscoating of the article itself with one diffusion material for alteringthe acceptance charac teristics of the article surface for another orprimary diffusion material).

As noted above, at least with the high-nickel and highcobalt superalloymaterials illustratively disclosed, satisfactory results are achievedwith a coating pack having some 69% or 70% inert filler (such asalumina) and diffusible metallic components in the amount of, forexample, 22% chromium and 8% aluminum to begin with, along with aneffective amount of about 1% vaporizable halide carrier. In commercialoperations, as will be understood, it is generally intended to reuse agiven quantity of coating pack again and again. Since each use of agiven quantity of pack exhausts some componentstat least the aluminumand carrier components) to some extent, effective reuse of the powderedpack ingredients requires some regeneration thereof (as, for example,adding an additional 20% of the metallic component thereto, along withadditional carrier component), so that the precise composition of thepack itself constantly changes, as a result of reaction exhausting ofreactant components thereof as well as the inevitable mechanical lossesof handling. As has been found, in accordance herewith,

that, although a starting pack composition is preferred as abovedisclosed, continued use and reuse of such a pack, with the necessaryreplenishment thereof, results in a certain equilibrium condition in thepack, which condition has been noted to be approached even afterutipeated uses of the pack ingredients and upon normal regenerationthereof with about 20% active ingredients. Similarly, although thetemperature and time ranges noted above may also be preferred as astandard commercial operating condition, it is to be understood that,especially with chromium-aluminum packs in the aluminizing treatment ofhigh-nickel and high-cobalt superalloys, satisfactory results are alsoobtained by holding the articles to be coated embedded in the pack attemperature (i.e., after heating the retort up to temperature) for atime range of approximately -%-40 hours within a temperature range ofabout l400-2200 F., depending, of course, on the particular materialbeing coated and the depth or thickness of coating case desired.

The foregoing, of course, relates to a situation where the particularcarrier material is selected specifically to attack and break down therate-controlling intermetallic formed in the pack ingredients, as notedabove, although the selection of a particular carrier material, with dueregard to its aggressiveness of activeness with the coating metal(whether or not preliminarily combined with another ingredient in thepack), as well as the quantity thereof, are also to be considered as arate-controlling technique within the disclosure above.

Accordingly, and as will understood from all of the foregoing, there areprovided herewith techniques and compositions for controlling thediffusion coating of a variety of coating materials into a variety ofcoated metal articles in a manner so that the intermetallic compounds 4or alloys formed in the surface of the article being coated may beadequately controlled and predicted notwithstanding the fact that thecharacteristics inherent in the coating pack and the constituentsthereof, as well as the diffusion or transfer rates for any particularoperating temperature, are different from or actually incompatible withthe conditions desired for the formation in the coated article of thespecifically and optimumly desired product; and the techniques andcompositions in accordance herewith satisfactorily accommodate suchinimical or incompatible situations for controlling the composition andactivity of the coating pack in a manner, perhaps, different than theinherent characteristics thereof for the purpose of controlling thecomposition of the ultimately desired product by inhibiting orcontrolling the rate or quantity at which the coating material ispresented or transferred from the powdered pack to the surface of thearticle being coated and, preferably, by the addition to the coatingpack of a combining reactant for producing the controlled transfereffect desired of the coating material to the surface of the articlebeing coated by or through the medium of a vaporizable carriercomponent.

While the methods and compositions herein disclosed form preferredembodiments of this invention, this invention is not limited to theseprecise methods and compositions and modifications thereof may beprovided without departing from the scope of this invention which isdefined in the appended claims.

What is claimed is:

1. In a method for the production of a diffusion coating of thecharacter described on the surface of an alloy base metal having hightemperature resistant characteristics and a substantial proportion ofapproximately half of a metal selected from the group consisting ofnickel and cobalt, and a substantial proportion of chromium, thev stepswhich comprise embedding said alloy base metal in a diffusion coatingpack including a source of chromium metal and sufficient aluminum metalfor effecting diffusion coating thereof into the surface of said basealloy and a source of vaporizable halogen as a carrier for said basemetal having high temperature resistant characteristics and having asubstantial proportion of approximately half of a metal of the groupconsisting of nickel and cobalt and a substantial proportion ofchromium, the steps which comprise embedding said alloy base metal in adiffusion coating pack including a source of chromium metal andsufficient aluminum metal for effecting diffusion coating thereof intothe surface of said base alloy and a source of vaporizable halogen as acarrier for said aluminum in said diffusion coating thereof and powderedfiller material, heating said base alloy in said pack to a temperatureof at least about 1800 F. effecting diffusion coating of said aluminuminto the surface of said article to produce said diffusion coating ofaluminum thereon said aluminum being present in an amount at least 3% byweight of said pack.

3. In a method for the production of a diffusion coating of thecharacter described on the surface of an alloy base metal having hightemperature resistant characteristics and having a substantialproportion of approximately half of a metal selected from the groupconsisting of nickel and fusion coating pack including a source ofchromium metaland sufficient aluminum metal for effecting diffusioncoating thereof into the surface of said base alloy and a source ofvaporizable halogen as a carrier for said aluminum in the diffusioncoating thereof and powdered filler material, heating said base alloy insaid pack effecting diffusion coating of said aluminum into the surfaceof said article, said chromium being present in said pack in a ratio ofabout 2-4 times by Weight the amount of aluminum therein.

4. In a method for the production of a diffusion coating of thecharacter described on the surface of an alloy base metal having hightemperature resistant characteristics and having a substantialproportion of approximately half of a metal of the group consisting ofnickel and cobalt and a substantial proportion of chromium, the stepswhich comprise embedding said alloy base metal in a diffusion coatingpack including a source of chromium metal and sufficient aluminum metalfor effecting diffusion coating thereof into the surface of said basealloy and a source of vaporizable halogen as a carrier for said aluminumin the diffusion coating thereof and powdered filler material, heatingsaid base alloy in said pack effecting diffusion coating of saidaluminum into the surface of said article, said aluminum comprisingabout 3%-20% of said pack by 'weight and said chromium being present inan amount about 2-4 times by weight the amount of said aluminum.

5. A metal article of the character described and susceptibleto longexposure to an oxidizing and corrosive atmosphere at high temperatureand resistant to the thermal shock incident to repeated heating andcooling between ambient temperature to high temperature, and comprisinga base alloy including a substantial proportion of aproximately half ofat least one of the metals selected from the group consisting of nickeland cobalt and a substantial proportion of chromium, which article isenclosed within a diffused outer layer case including aluminum. and saidouter layer case having been formed on said article in accordance withthe method recited in claim 1.

6. A metal article as recited in claim 5 in which said outer layer caseis at least about 0.00l"0.002" thick over said article.

7. In a method for the production of a diffusion coating of thecharacter described on the surface of an alloy base metal having hightemperature resistant characteristics and a substantial proportion ofapproximately half of a metal selected from the group consisting ofnickel and cobalt, and a substantial proportion of chromium, the stepswhich comprise embedding said alloy base metal article in a diffusioncoating pack including a source of chromium metal and sufiicientaluminum metal for eflfecting diffusion coating thereof into the surfaceofsaid base alloy and a source of vaporizable halogen as a carrier forsaid aluminum in said diffusion coating thereof, the weight ratio ofsaid chromium to said aluminum in said powdered pack being substantiallywithin the range of about 0.5 to 4.6, and heating said basemetal alloyin said pack effecting diffusion coating of said aluminum into thesurface of said article.

8. A method as recited in claim 7 in which said heating step isaccomplished at temperatures within the range of about 1400-2200 F. andprolonged for a time at said temperature within the range of about 4-40hours.

9. In a diffusion coating process where a metallic coating is diffusedinto the surface of a metal article by heating such metal article in anon-oxidizing atmosphere in a sealed powdered diffusion coating packincluding the metallic coating material to be diffused into said articleand a carrier component for effecting the transfer of said metalliccoating material from said pack to the surface of said article, thesteps which comprise incorporating in said diffusion coating pack anadditional metallic ingredient for combining chemically with saidcoating material in said pack for inhibiting and controlling saidtransfer thereof to the surface of said article to be coated, effectingsaid chemical combination with said additional ingredient, andtransferring said coating material from said chemical combination to thesurface of said article for said diffusion coating thereinto of onlysaid coating material and substantially in the absence of diffusion ofsaid additional metallic ingredient into said article.

10. A process as recited in claim 9 in which said metallic coatingmaterial to be diffused into said article is selected from the groupconsisting of aluminum, chromium, iron, silicon, and mixtures thereof.

11. A process as recited in claim 9 in which said metal article to becoated comprises a substantial proportion of a metal selected from thegroup consisting of chromium, cobalt, copper, iron, nickel, therefractory metals, and alloys and mixtures thereof.

12. A process as recited in claim 9 in which said additional metallicingredient added to said pack for combining chemically with said coatingmaterial comprises a material selected from the group consisting ofaluminum, beryllium, boron, carbon, chromium, cobalt, iron, nickel, therefractory metals, silicon, and mixtures and alloys thereof.

13. A process as recited in claim 9 in which said metal article to becoated comprises a substantial proportion of a metal selected from thegroup consisting of chromium, cobalt, copper, iron, nickel, therefractory metals, and alloys and mixtures thereof; and in which saidmetallic coating material to be diffused into said article is selectedfrom the group consisting of aluminum, chromium, iron, silicon, andalloys and mixtures thereof; and in which said additional metallicingredient added to said pack for combining chemically with said coatingmaterial is selected from the group consisting of aluminum, beryllium,boron, carbon, chromium, cobalt, iron, nickel, the refractory metals,silicon, and alloys and mixtures thereof.

14. In a method for the production of a diffusion coating of thecharacter described on the surface of an alloy base metal having hightemperature resistant characteristics and a substantial proportion ofapproximately half of a metal selected from the group consisting ofnickel and cobalt, and a substantial proportion of chromium, the stepswhich comprise embedding said alloy base metal article in a diffusioncoating pack including a source of sufficient aluminum for effectingditfusion coating thereof into the surface of said base alloy and asource of vaporizable halogen as a carrier for said aluminum in saiddiffusion coating thereof and an additional separate metallic ingredientfor combining chemically with said aluminum in said pack for inhibitingand controlling said transfer thereof to the surface of said alloy basemetal, and heating said alloy base metal in said pack effectingdiffusion coating of said aluminum only into the surface of said articlesubstantially in the absence of diffusion of said separate metallicingredient thereinto.

References Cited by the Examiner UNITED STATES PATENTS 1,899,569 2/1933Howe. 2,536,774 1/ 1951 Samuel. 2,811,466 10/1957 Samuel 117-222,837,442 6/1958 Seelig et al. 2,874,070 2/1959 Galrniche 117-10722,875,090 2/1959 Galmiche 117-1072 X 3,061,462 10/ 1962 Samuel.3,073,015 1/1963 Wachtell et al. 3,079,276 2/1963 Puyear et al. 117-10723,096,160 7/1963 Puyear et al. 3,096,205 7/1963 De Guisto 117-10723,108,013 10/1963 Pao Jen Chao et al. 117-1072 3,117,846 1/1964 Pao JenChao 117-1072 X FOREIGN PATENTS 586,241 3/1947 Great Britain.

685,683 1/1953 Great Britain.

722,797 2/ 1955 Great Britain.

749,056 5/1956 Great Britain.

ALFRED L. LEAVITT, Primary Examiner. RICHARD D. NEVIUS, Examiner. R. S.KENDALL, Assistant Examiner.

9. IN A DIFFUSION COATING PROCESS WHERE A METALLIC COATING IS DIFFUSEDINTO THE SURFACE OF A METAL ARTICLE BY HEATING SUCH METAL ARTICLE IN ANON-OXIDIZING ATMOSPHERE IN A SEALED POWDERED DIFFUSION COATING PACKINCLUDING THE METALLIC COATING MATERIAL TO BE DIFFUSED INTO SAID ARTICLEAND A CARRIER COMPONENT FOR EFFECTING THE TRANSFER OF SAID METALLICCOATING MATERIAL FROM SAID PACK TO THE SURFACE OF SAILD ARTICLE, THESTEPS WHICH COMPRISE INCORPORATING IN SAID DIFFUSITON COATING PACK ANADDITIONAL METALLIC INGREDIENT FOR COMBINING CHEMICALLY WITH SAIDCOATING MATERIAL IN SAID PACK FOR INHIBITING AND CONTROLLING SAIDTRANSFER THEREOF TO THE SURFACE OF SAID ARTICLE TO BE COATED, EFFECTINGSAID CHEMICAL COMBINATION WITH SAID ADDITIONAL INGREDIENT, ANDTRANSFERRING SAID COATING MATERIAL FROM SAID CHEMICAL COMBINATION TO THESURFACE OF SAID ARTICLE FOR SAID DIFFUSION COATING THEREINTO OF ONLYSAID COATING MATERIAL AND SUBSTANTIALLY IN THE ABSENCE OF DIFFUSION OFSAID ADDITIONAL METALLIC INGREDIENT INTO SAID ARTICLE.